Problem: A circle has a circumference of $4\pi$. It has an arc of length $1\pi$. What is the central angle of the arc, in degrees? ${4\pi}$ ${90^\circ}$ $\color{#DF0030}{1\pi}$
Explanation: The ratio between the arc's central angle $\theta$ and $360^\circ$ is equal to the ratio between the arc length $s$ and the circle's circumference $c$ $\dfrac{\theta}{360 ^ \circ} = \dfrac{s}{c}$ $\dfrac{\theta}{360 ^ \circ} = 1\pi \div 4\pi$ $\dfrac{\theta}{360 ^ \circ} = \dfrac{1}{4}$ $\theta = \dfrac{1}{4} \times 360 ^ \circ$ $\theta = 90^\circ$